Contact hole production is a common step in semiconductor integrated circuit manufacturing. The contact holes are typically used to make electrical connections to a semiconductor or metal layer through an overlying non-conducting (dielectric) layer, such as an oxide layer, or partially-conductive layer. In order to produce contact holes, a layer of photoresist is deposited on the wafer surface. The photoresist is exposed to ultraviolet or other radiation, hardened and developed in order to form a “mask” over the wafer, with openings at the locations of the contact holes. Then the wafer is transferred to an etch station to form the contact holes through the non-conducting layer down to the semiconductor layer. The photoresist mask is then removed, and the contact holes are filled with metal. A similar process is used in producing trenches or vias in the wafer surface.
In order to ensure consistent device performance, the depth, width and bottom surface cleanliness of contact openings must be carefully controlled. (In the context of the present patent application and in the claims, the term “contact openings” refers to all structures of the type described above, including both contact holes, vias and trenches. Certain techniques for inspecting contact openings and monitoring their production, however, are described by way of example with specific reference to contact holes.) Deviations in the dimensions of contact openings can lead to variations in the contact resistance. These variations can have a serious impact on device performance and can lead to loss of process yield. The manufacturing process must therefore be carefully monitored and controlled, in order to detect deviations in formation of contact openings as soon as they occur and to take corrective action to avoid the loss of costly wafers in process.
It is known in the art to use a scanning electron microscope (SEM) to inspect contact holes and other contact openings. The principles of the SEM and its use in microanalysis of semiconductor device structures are described, for example, by Yacobi et al., in Chapter 2 of Microanalysis of Solids (Plenum Press, New York, 1994), which is incorporated herein by reference. Because contact holes are typically much deeper than they are wide, a special high aspect ratio (HAR) imaging mode is used, as described by Yacobi et al. Open contact holes, which reach down through the dielectric layer to the semiconductor below, appear bright in the image, while closed holes, which do not fully expose the semiconductor layer, are dim.
HAR techniques using a SEM are time-consuming and costly to implement, and they become impractical at very high aspect ratios (roughly >10), which are used in some integrated circuits, such as DRAM. They are also not capable of distinguishing between different types of blockage that can cause contact holes to be closed (for example, under-etching of the holes, as opposed to deposition of residues in the bottoms of the holes). Furthermore, HAR imaging techniques can generally be used only after the photoresist mask has been cleaned from the wafer surface. Consequently, there is no possibility of continuing the etching process if it is discovered upon inspection that the contact holes have been underetched.
An alternative method for contact hole inspection is described by Yamada et al., in “An In-Line Process Monitoring Method Using Electron Beam Induced Substrate Current,” in Microelectronics-Reliability 41:3 (March 2001), pages 455-459, which is incorporated herein by reference. The substrate current in an electron beam system, also known as the specimen current, absorbed current or compensation current, is defined as the absorbed current that flows or would flow from the primary electron beam to ground (earth) via the specimen (i.e., via the wafer). In other words, the specimen current is equal to the difference between the primary beam current (i.e., the current of electrons in the electron beam that irradiates the specimen in the system) and the total yield of electrons emitted from the surface of the specimen due to secondary and backscattered electrons (adjusted for any local charging effects or time constants). The specimen current can be either positive or negative, depending on whether the energy of the primary electron beam is in the positive- or negative-charging domain of the specimen. (The phenomena of positive and negative charging by e-beam irradiation are described in the above-mentioned reference by Yacobi et al.) Yamada et al. directed an electron beam at single contact holes and groups of holes in a SiO2 surface layer overlying a silicon substrate, and measured the resultant specimen current. They found that the specimen current was a good indicator of hole-bottom oxide thickness, as well as of the hole diameter.
Yamada et al. describe further aspects of contact hole measurement in U.S. patent application Publication No. US 2002/0070738 A1, whose disclosure is incorporated herein by reference. Semiconductor devices are inspected by measuring the specimen current in an area of a sample having no contact holes as a background value, and comparing this value to the current measured in the area of a hole. A current waveform is automatically evaluated in order to determine whether the measurement is indicative of a defect of the device or of manufacturing equipment used in producing the device.